In photolithography, a desired pattern is applied onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material, usually referred to as a resist, which is provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured. A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ            NA                                              (        1        )            where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm. Such radiation is sometimes termed soft x-ray radiation. EUV radiation may be produced using a plasma. A radiation system for producing EUV radiation may include a laser for exciting a fuel to provide the plasma, and a source collector module for containing the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as particles of a suitable material (e.g. tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. Such a radiation system is typically termed a laser produced plasma (LPP) source. Alternative sources include discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.
In EUV lithography, the choice of wavelengths may be limited by practical considerations involving the availability of suitable radiation sources, optical components and process materials. Current EUV lithography systems all operate using radiation wavelength within the range of 13-14 nm, and many developments remain to be made before EUV lithography is used in volume production. It has further been proposed that EUV radiation with a wavelength of less than 11 nm could be used, for example within the range of 5-10 nm or 5-8 nm, and especially in the so-called ‘6.x’ wavelength region of 6.5-6.9, for example 6.7 or 6.8 nm. The intention is that the shorter wavelength may provide a better resolution (features below the 11 nm node), larger depth of focus (DOF) and higher throughput compared to the 13.5 nm radiation that is currently used. However, the change of wavelength brings a new range of practical considerations and the techniques and materials optimized for 13.5 nm may or may not work at the shorter wavelengths.
A particular challenge for the development of commercial EUV lithography lies in the formulation of radiation-sensitive resist materials that will realize, in etch-resistant material, a high resolution pattern projected by the EUV optical system. Some work has been published concerning the development of resist materials usable at 13.5 nm. The inventors have recognized that quite different solutions may be appropriate for use at shorter wavelengths below 11 nm.